In the construction machine such as the hydraulic excavator, an engine is mounted to a vehicle body, and a working mechanism including a swiveling apparatus, and a boom and the like is actuated on the basis of rotation of the engine via a hydraulic pump or the like. Further, a cooling fan and a hydraulic motor for rotating the cooling fan are mounted to the vehicle body, the hydraulic motor is rotated via the hydraulic pump in accordance with rotation of the engine, and the cooling fan is rotated and blows air. On the basis of the rotation, a working fluid such as a cooling water for cooling the engine and hydraulic oil for actuating the hydraulic motor is cooled.
With respect to the construction machine mentioned above, if the cooling fan is directly coupled to the engine, the cooling fan is rotated at a rotating speed of a fixed relation generated together with the rotating speed of the engine. Accordingly, even in the case that a temperature of the hydraulic oil or the like is low and it does not need to be cooled, the cooling fan is rotated at a high speed as the engine is rotated at a high speed. Therefore, an overcooled state is created, and an engine output is wastefully consumed. Further, there is a problem that an ambient noise is increased in accordance with the rotation of the cooling fan.
In order to cope with the problems mentioned above, construction machines have been widely used that are structured such that the rotating speed of the cooling fan can be independently controlled by rotating the cooling fan by the hydraulic motor. Further, in construction machines disclosed in Patent Document 1 and Patent Document 2, in the case that a temperature of a hydraulic oil or the like is low and it does not need to be cooled, the flow rate of the hydraulic oil is controlled by a valve such that the flow rate of the hydraulic oil discharged from a hydraulic pump for a fan is reduced. Accordingly, the rotating speed of the cooling fan is lowered, the load of an engine is reduced, and it is possible to effectively utilize the output of the engine to drive a main hydraulic pump or the like that actuates a boom or the like. In addition thereto, ambient noise generated by the rotation of the cooling fan is reduced on the basis of the reduction of the rotating speed of the cooling fan.
However, in the conventional structure mentioned above, there has been a problem that a resonance is generated between the cooling fan and the engine at a time when the rotating speed of the cooling fan comes to a specific relation to the rotating speed of the engine due to the reduction of the rotating speed of the cooling fan, whereby great noise and vibration are generated.
Specifically, a relation of an expression (1) in a mathematical expression 1 is established when the engine and the cooling fan rotate. When the difference in frequency between the noise due to the primary frequency of the fan and the noise due to the primary frequency of the engine comes to within a specific value, for example, 10 Hz, the noises resonate and great and uncomfortable resonance noise and vibration are generated. Particularly, in a state in which all of working machine levers 18 are not operated but are arranged at a neutral position, various devices such as a hydraulic pump and the like constituting the working machine are not operated. Accordingly, the noise and the vibration generated together with the resonance noise and the resonance become remarkable, so that an uncomfortable feeling doubles. The value of the frequency difference between the fan primary frequency noise and the engine primary frequency noise, which difference generates resonance, somewhat varies in accordance with a type of motor vehicle and the presence of noise absorbing material.
Expression 1Δf=|Ne×(C/2)/60−Nf×F/60|  (1)Δf: frequency difference between engine and fan (Hz)Ne: engine rotating speed (rpm)C: number of engine cylindersNf: fan rotating speed (command value) (rpm)F: number of fan impeller bladesPatent Document 1: Japanese Laid-Open Patent Publication No. 2000-110560Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-83344